Abstract/Summary

The Silesian (Upper Carboniferous) sedimentary rocks of the Culm Basin, to the west of Tiverton, belong to the Crackington and Bude Formations. These comprise
turbidite and debrite sandstone, interbedded with mudstone. In the field the
formations are differentiated on the basis of thickness and weathering characteristics
of the sandstone packages.
In the study area although bedrock is commonly obscured by overlying superficial
deposits, the character of these deposits can be used as an indicator of bedrock
geology. Using both bedrock and superficial analytical techniques, the Crackington /
Bude boundary has been mapped in the study area. Tracing of large scale structure
has been helped by the use of shaded relief digital terrain models, however in areas of
moorland the distinctive featuring is not present and there are no indicators of the
underlying bedrock structure.
Geophysical data, in particular a recent gravity survey, provide considerable
information on the structure of the Culm Basin. It is clear that the east-west trend of
the Tiverton Trough, depicted by a negative gravity anomaly over the Permo-Triassic
infill, continues westwards in the Silesian sedimentary rocks. Further west of the
Tiverton Trough an area of relatively high gravity covers the area of Witheridge and
Rackenford moors. North of this is a negative gravity anomaly whose northern
boundary lies close to the crop of the basal Crackington Formation and whose
southern margin follows the Crackington / Bude boundary. The strong east-west
linear nature of the gravity anomalies indicates that the structure controlling the
northern boundary of the Tiverton Trough was probably active during Variscan
inversion. The gravity high over Witheridge is thought to be related to the presence of
dense, earlier Palaeozoic rocks that form a structural high underlying the Silesian;
given its linear nature and its alignment with the northern boundaries of both the
Tiverton and Crediton troughs.